DESCRIPTION (Applicant's Description): Segmentation subdivides tissues into a series of repeating units along either the body axis, or in some cases, the appendage axis. This process is fundamental to the organization of most animal bodies. In most cases, segmentation must occur repeatedly as tissues grow in size. The molecular mechanisms involved in generating repeated segmentation in growing tissues are not well understood. The long-term goal of this proposal is to elucidate the cellular and molecular mechanisms of segmentation in the Drosophila leg, a model tissue in which segmentation is coordinated with growth. The Notch signaling pathway plays a key role in the segmentation and growth of the Drosophila leg. Notch activation in the leg is positioned by the segmentally patterned expression of its ligands and the Notch pathway modulator fringe. The first aim of this proposal is to identify and characterize mechanisms that are involved in establishing the segmental patterns of Notch ligand expression. The second aim of this proposal is to identify and characterize mechanisms that are involved in maintaining, restricting, and modifying segmentally repeated patterns of Notch ligand expression as the leg grows. The third aim of this proposal is to identify and characterize targets of Notch signaling that are responsible for affecting its influence on the actual growth and segmentation of the leg. The Notch signaling pathway plays essential roles in the development and function of a wide range of metazoan tissues, and mutations in human Notch pathway genes have been implicated in leukemia (TAN-1), stroke and dementia (CADASIL), and Alagille syndrome, a childhood syndrome resulting in chronic liver disease and segmentation defects. Thus, the proposed experiments have the potential to provide insights into the regulation and functioning of the Notch pathway that will be broadly relevant to understanding cancer and other human disease states. The proposed experiments employ a range of cellular, molecular, and genetic techniques to analyze patterns of gene expression and gene function in Drosophila legs. The expression of candidate regulators of Notch ligand expression will be manipulated, and the consequences of these manipulations will be assayed by immunostaining. Similarly, the influence of known and candidate downstream target genes of Notch signaling on leg growth and segmentation will also be assessed by using standard molecular and genetic techniques to manipulate their expression.